1. Field of the Invention
The present invention relates in general to processing a video signal in a television (referred to hereinafter as TV) receiver, and more particularly to a chrominance signal processing apparatus for a digital TV receiver which is capable of processing a chrominance signal using an automatic color control (referred to hereinafter as ACC) circuit and a color killer circuit.
2. Description of the Prior Art
In the National Television System Committee (NTSC) system, generally, a black-and-white TV receiver can produce a black-and-white picture upon receiving a color broadcasting signal, whereas a color TV receiver can produce a color picture when it receives the color broadcasting signal. The color TV receiver may also obtain the black-and-white picture by changing its channel to a black-and-white broadcasting channel.
In detail, in the NTSC system, primary color signals R, G and B of red, green and blue are converted into a luminance signal Y and color difference signals I and Q and then transmitted. Upon receiving the transmitted luminance signal Y and color difference signals I and Q, the black-and-white TV receiver produces the black-and-white picture in response to the received luminance signal Y. When the color TV receiver receives the transmitted luminance signal Y and color difference signals I and Q, it detects the primary color signals R, G and B from the received luminance signal Y and color difference signals I and Q and produces the color picture in response to the detected primary color signals R, G and B.
FIG. 1 is a view illustrating one line of a composite video signal in the NTSC system. As shown in this drawing, a chrominance signal and a color burst signal are applied to a luminance signal. A horizontal synchronous signal has its amplitude of the range from 0 IRE to -40 IRE. The color burst signal (CBS) includes at least 8 cycles of chrominance subcarrier inserted in a back porch of the horizontal synchronous signal. The chrominance signal has its amplitude of the range from 0 IRE to 100 IRE. The color burst signal CBS has a peak to peak value (P--P) of the range from -20 IRE to +20 IRE.
Noticeably, the amplitude of the chrominance signal may be varied due to distortions resulting from a noise and an interference during its transmission, distortions occurring at an antenna and a receiver, a channel change and etc. In order to compensate for such an amplitude variation of the chrominance signal, there has been employed an ACC circuit.
On the other hand, in the case where a black-and-white video signal is received by the color TV receiver, it is imperfectly processed by a luminance/chrominance separation circuit, resulting in the occurrence of cross-color and cross-luminance phenomenons. For this reason, the amplitude of the chrominance signal is checked to discriminate whether the input video signal is black-and-white. If it is discriminated that the input video signal is black-and-white, a color killer circuit is used to block the output of the chrominance signal.
The ACC circuit and the color killer circuit are adapted to determine the amplitude of the chrominance signal on the basis of the amplitude of the color burst signal which has the peak to peak value (P--P) of the range from -20 IRE to +20 IRE. Noticeably, the amplitude of the color burst signal may be increased or reduced due to the distortions during its transmission and at the antenna and receiver. For this reason, the ACC circuit and the color killer circuit serve to control the amplitude of the chrominance signal on the basis of the amplitude of the color burst signal.
Referring to FIG. 2, there is shown a block diagram of a conventional chrominance signal processing apparatus for an analog TV receiver. As shown in this drawing, the conventional chrominance signal processing apparatus comprises a first band pass filter/amplifier circuit 11 for detecting a chrominance signal component of subcarrier frequency band (fs: 3.58 Mhz) from a chrominance signal C from a luminance/chrominance separation circuit (not shown) in the analog TV receiver, a burst gate 12 for detecting a color burst signal CBS from the chrominance signal from the first band pass filter/amplifier circuit 11 in response to an external burst gate signal BGS, an ACC circuit 13 for detecting an amplitude of the color burst signal CBS from the burst gate 12 and comparing the detected amplitude with a reference level, and a color killer circuit 14 for determining in response to an output signal from the ACC circuit 13 whether an input video signal is color or black-and-white and outputting a control signal in accordance with the determined result.
The conventional chrominance signal processing apparatus further comprises a second band pass filter/amplifier circuit 15 responsive to the control signal from the color killer circuit 14 for passing the chrominance signal from the first band pass filter/amplifier circuit 11 if the input video signal is color and blocking it if the input video signal is black-and-white, a chrominance demodulator 16 for detecting color difference signals I and Q from the chrominance signal passed by the second band pass filter/amplifier circuit 15, and a matrix unit 17 for detecting primary color signals R, G and B from the color difference signals I and Q from the chrominance demodulator 16 and a luminance signal Y from the luminance/chrominance separation circuit and outputting the detected primary color signals R, G and B to a monitor.
The operation of the conventional chrominance signal processing apparatus for the analog TV receiver with the above-mentioned construction will hereinafter be described with reference to FIGS. 1 to 3.
First, the composite video signal as shown in FIG. 1 is transmitted to the analog TV receiver. The luminance/chrominance separation circuit in the analog TV receiver separates the chrominance signal C and the luminance signal Y from the transmitted composite video signal. The luminance/chrominance separation circuit then supplies the separated chrominance signal C and luminance signal Y to the first band pass filter/amplifier circuit 11 and the matrix unit 17, respectively. The first band pass filter/amplifier circuit 11 detects the chrominance signal component of subcarrier frequency band (fs: 3.58 Mhz) as shown in FIG. 3 from the chrominance signal C from the luminance/chrominance separation circuit. The first band pass filter/amplifier circuit 11 then amplifies the detected chrominance signal by a predetermined level and applies the amplified chrominance signal to the second band pass filter/amplifier circuit 15 and the burst gate 12.
The burst gate 12 detects the color burst signal CBS from the chrominance signal from the first band pass filter/amplifier circuit 11 in response to the external burst gate signal BGS and outputs the detected color burst signal CBS to the ACC circuit 13. The ACC circuit 13 detects the amplitude of the color burst signal CBS from the burst gate 12 and compares the detected amplitude with the reference level (-20 IRE-+20 IRE). The ACC circuit 13 then outputs a signal as a result of the comparison to the first band pass filter/amplifier circuit 11 and the color killer circuit 14.
At this time, the first band pass filter/amplifier circuit 11 serves to control an amplitude of the chrominance signal in response to the output signal from the ACC circuit 13. Namely, when the amplitude of the color burst signal CBS is lower than the reference level (-20 IRE-+20 IRE), the first band pass filter/amplifier circuit 11 increases the amplitude of the chrominance signal. On the contrary, when the amplitude of the color burst signal CBS is higher than the reference level (-20 IRE-+20 IRE), the first band pass filter/amplifier circuit 11 reduces the amplitude of the chrominance signal.
The color killer circuit 14 serves to control the output of the second band pass filter/amplifier circuit 15 in response to the output signal from the ACC circuit 13. When the amplitude of the color burst signal CBS is lower than the reference level (-20 IRE-+20 IRE), namely, when the input video signal is black-and-white, the color killer circuit 14 controls the second band pass filter/amplifier circuit 15 to block the output of the chrominance signal therefrom.
As a result, when the input video signal is black-and-white, the second band pass filter/amplifier circuit 15 outputs a value of "0" in response to the control signal from the color killer circuit 14. On the contrary, when the input video signal is color, the second band pass filter/amplifier circuit 15 passes the chrominance signal from the first band pass filter/amplifier circuit 11 to the chrominance demodulator 16.
The chrominance demodulator 16 detects the color difference signals I and Q from the chrominance signal passed by the second band pass filter/amplifier circuit 15 and supplies the detected color difference signals I and Q to the matrix unit 17, which also receives the luminance signal Y which is fed from the luminance/chrominance separation circuit and then delayed to match with the color difference signals I and Q from the chrominance demodulator 16. The matrix unit 17 combines the received color difference signals I and Q with the received luminance signal Y and produces the primary color signals R, G and B in accordance with the combined result. The matrix unit 17 then outputs the produced primary color signals R, G and B to the monitor so that a color picture can be displayed on a screen of the monitor.
However, the ACC circuit and the color killer circuit employed in the chrominance signal processing apparatus for the analog TV receiver cannot be applied to a digital TV receiver. Embodying the ACC circuit and the color killer circuit in a digital manner requires the use of many multipliers and read only memories (referred to hereinafter as ROMs), resulting in an increase in the cost and a difficulty in the integration.